London: Amid the hubbub of the city, the sound of a sparrow singing can be a welcome reminder of a more peaceful time. Today’s urban sparrows have a far more high-pitched song than they did 50 years ago because they want to be heard over noise of the city, experts say.

American researchers compared archived recordings of sparrow song from the 1960s with tapes made in the same area of San Francisco in 2005.

They also looked at data on noise levels in the city and the number of cars crossing the Golden Gate Bridge.

Professor David Luther discovered modern sparrows are shriller than their ancestors, and that the number of songs or "dialects" sung in the area has fallen.

In the 1960s, the white-crowned sparrows studied sung three dialects. Thirty years later that had fallen to two and today only one dominates, known as the San Francisco dialect.

It is the highest-pitched of the three, and the researchers believe it was the easiest one to adapt so it could be heard over background noise.

Luther of George Mason University in Virginia said that, while other studies have looked at the differences between city and country birdsong, he published in his Journal Animal Behavior, is the first to track songs in one area over time.

In a previous study, scientists found that great tits in noisier parts of the Netherlands have higher-pitched songs than those in more peaceful areas.

Another found that Berlin's nightingales sing up to 14 decibels louder than those in nearby forests. The dawn chorus is actually getting louder as city birds sing louder to be heard over noise such as speed cars and car horns.